<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>三角形</title>
</head>

<style>

    .box {
        margin: auto;
        margin-top: 200px;
        width: 0;
        height: 0;
        border: 50px solid transparent;
        border-left-color: red;
    }

</style>
<body>
    <div class="box"></div>
</body>
</html>